Metallicity in Astronomy

In chemistry, when they talk about metals, they mean anything that is generally on the left side of the periodic table. However, in astronomy, metals are something completely different.


Astronomers break down elements into three groups: hydrogen, helium, and metals. So anything that is not hydrogen or helium is considered a metal. This includes non-metallic elements like carbon, oxygen, and nitrogen. Why?


In the beginning of the universe, the universe was composed of 75% hydrogen (basically, bare protons) and 25% helium. Over time, as the universe has evolved, there has been trace materials heavier than helium created in stars. Now, the ratio is still relatively the same, but with trace other elements. To make it easier to describe stuff, astronomers use the generic term metals.


So one of the numbers that are used to describe stars is something called the metallicity of the star. In general, the younger the star, the more metals are found in the spectrum. The older stars have less metals because they formed when there were less metals in the universe. For example, our Sun has a metallicity of Z=0.0122. What does this number mean?


The metallicity is the mass fraction of the amount of metals in the object. For the Sun, its mass is only 1.22% of its total mass. You can also compare the metallicity of a star to that of the Sun.


What astronomers use is the amount of iron in a star and compare that to the amount of iron in the Sun. This is defined as [Fe/H] which is the difference in the base 10 logarithms of the ratios of iron to hydrogen in the star and in the Sun. Iron is used as the spectral lines of iron are easy to see in stellar spectra.



For a star older than the Sun, it is possible for [Fe/H] to be negative. For the Sun, you can easily see that [Fe/H] is exactly 0. For younger stars, [Fe/H] can be greater than 0. We can determine the amount of iron in the star and the Sun by the strength of the spectral lines of iron.

The same thing can be calculated for any element in a star.